Steam line isolation valves of this kind, also known as reheat stop valves, are a safety device. They are provided before the entry of the steam into-the low-pressure turbines downstream of the first turbine cylinder in saturated steam turbo sets if the overspeed occurring in the event of load shedding of the system cannot be limited to permissible values in any other way. In the event of load shedding as the result of a three-phase line fault, for example, the load torque of a generator driven by the turbo set quickly disappears. In this case the main steam valves are closed so as to prevent further steam from being supplied to the first turbine cylinder. However, the steam still stored in this turbine cylinder, the intervening steam lines and any moisture separator or reheater continues to expand. Because of the absence of load torque, the expansion causes the speed of the turbo set to increase. It is therefore necessary to prevent this expansion and to prevent steam from entering the second and any other turbine cylinders. A completely leak-tight isolation is not necessary. Small leaks can be tolerated.
U.S. Pat. No. 3,444,894 discloses a device for controlling the pressure or the quantity of a gaseous medium. The device has a housing which defines a longitudinally extending channel and has an inlet port and an outlet port for the medium. Two so-called damping paddles are disposed in the housing and can be moved against one another vertically with respect to the longitudinal axis. In addition, a central element is disposed essentially centrally in the channel between the damping paddles. The central element is streamlined for favorable flow and extends along the longitudinal axis in the channel. At its upstream end it has a round profile of appreciable thickness, whereas it runs to a point at its downstream end.
DE 36 07 736 C2 describes a shutoff valve for pipework and the like whose housing contains a swivel-mounted valve which in its closed position bears on the inside of a seal lining disposed continuously over the entire housing width and made of a rigid or only slightly flexible plastic such as a fluoroplastic. In the sealing area, in which it has a slightly smaller clear diameter compared to the valve in the open position, the seal lining is compliantly disposed toward the closed position of the valve via a spring bridge and a gap between spring bridge and housing, the spring bridge, which has slots, being permanently fixed in the seal lining by partial or complete encasing, and the seal lining forming a unit with the spring bridge.
DE 38 26 592 A1 discloses an arrangement for actuating a stop valve in a steam line, preferably a steam line of a steam turbine. On a rotating shaft of the stop valve there is disposed a pinion with which two pairs of racks are engaged. One pair of racks is used in conjunction with hydraulic means for opening the stop valve, the other pair in conjunction with closing springs for rapid closing. By ensuring zero backlash, the two separate systems for opening and closing reduce mechanical wear and, via appropriate hydraulic circuitry, allow damping of the disk of the stop valve when it assumes the closed position. In order to maintain this damping irrespective of different operating states, manometric balances are used in conjunction with an interceptor throttle which can be adjusted as a function of the rotation angle. To relieve the pressure on the stop valve at opening, a bypass line is used which can in turn be shut off by fast-closing shutoff valves.
In the case of the known steam line isolation valves, a single valve is provided which is rotated to close the steam line. The pressure in the steam line is generally between 10-15 (18) bar for a diameter of 1.2 to 1.4 m. The closing time of the steam line isolation valve must be between one and two seconds. Because of the high stress due to the pressure, the steam line diameter and the temperatures obtaining, the valves must be of comparatively sturdy design. They are therefore very large and very heavy, resulting in a high moment of inertia about the rotational axis provided. To achieve the short closing time required, considerable acceleration torque therefore has to be applied to the valve.
Increasing the diameter of the valves currently in use is very difficult to achieve in terms of mechanical design. Drives capable of applying the required acceleration torques must first be provided. Difficulties in implementing the valve seating may also arise. Increasing the diameter would be desirable, however, as the entire cross-sections of the steam lines between the individual turbine cylinders can no longer be shut off at the current outputs of steam turbine systems. The steam line isolation valves must therefore be disposed in the supply lines to the individual second turbine cylinders. A separate steam line isolation valve is then necessary for every second turbine cylinder. This results in a high mechanical design complexity and financial outlay and an increased space requirement.